Telephone exchange system



Jan. 7, 1936.

s. w. ALLISON ET AL TELEPHONE EXCHANGE SYSTEM Filed Dec. 7, 1933 2Sheets-Sheet 1 N O S v I \\J\ U- A \ww Q d E 34$: p m ENExsK m6 H N P EQ5 V w 0% b5 W 4 ENEEE U mi U@ 5 j w wm m B.MC KIM ATTORNEY Jan. 7,1936. s. w. ALLISON ET AL TELEPHONE EXCHANGE SYSTEM Filed na 7. 1933 2Sheets-Sheet 2 ATTORNEY Patented Jan. 7, 1936 UNITED STATES TELEPHONEEXCHANGE SYSTEM Sewell W. Allison, West Orange, N. J., and James B. McKim, Douglas Manor, N. Y., assignors to Bell Telephone Laboratories,Incorporated, New York, N. Y., a corporation of New York ApplicationDecember 7, 1933, Serial No. 701,328

15 Claims.

This invention relates to improvements in telephone exchange systems andparticularly to trunking systems between exchanges.

More specifically the invention relates to circuit arrangements adaptedfor use with long toll switching trunks, employing reverse batterysupervision, extending from a toll operator in one exchange to a localor switching operator at a distant exchange.

An object of the invention is to control from the toll position, bymeans of direct current over the trunk line, the application of ringingcurrent to a called subscribers line at the local exchange therebyeliminating clicks, acoustic shocks, and the like frequently experiencedwith alternating circuit ringing.

Another object is to filter out the higher harmonies in the ringingcurrent so applied at the local exchange in order to minimize annoyanceto the called subscriber in case his receiver is connected to the lineand held to the car during the ringing period. Another object is topermit a ringing signal to be transmitted over the trunk either afterthe trunk line has been connected to the called line, or immediatelyupon seizure of the trunk at the toll ofiice and before the called linehas been connected to.

Other objects of the invention will be apparent from the followingdescription.

A feature whereby the first object is attained resides in introducing aringing control relay winding in series with the signaling batterysource and line and normally neutralizing the effect of this winding bymeans of an auxiliary winding in a local circuit which includes the samebattery. When the ringing signal is applied to the trunk the resistanceof the circuit including the line winding of the relay is increasedwhich reduces the flux caused by the iine windings whereupon the relayoperates due to the greater flux exerted by the auxiliary orcompensating winding. p A feature whereby the second object is attainedresides in connecting the ringing current source to the called linethrough the called line side of the trunk repeating coil at the localexchange. This winding acts as a filter to eliminate the harmonics ofthe ringing frequency.

Features whereby the other objects are attained will be understood fromthe following description when read in connection with the acgethercompanying drawings, which, when placed towith Fig. l at the top;represent a corn action from a toll cities to a called subscriber via atoll switching trunk with its-common position equipment, in accordancewith the invention.

It will be assumed that a. toll operator (A) has received a requesttocompleteaconnection to subscriber (X) and has inserted the answeringend of her cord into the calling trunk or toll line (not shown). lhetoll operator then proceeds to extend the connection by inserting thecalling plug 2 of cordl in jack 3 of the switching trunk embodying thepresent invention, whereupon sleeve relay 4 of the trunk operates in an'obvious circuit to close its contacts 5 which connect condenser 6between the right hand windings of repeating coil 1 and at its contacts8 bridges relay 9 across condenser 6 to operate line relay ID at thedistant end of the trunk in a circuit which can be traced from ground,compensating resistance ll (Fig. 2) left hand winding of line relay l0,middle winding of ringing relay l2, conductor l3, contacts M ofreversing relay i5, lower winding of retardation coil 16, trunkconductor ll, lower right hand winding of repeating coil 1, contacts 5and 8 of relay 4, winding of polarized supervisory relays, contacts E8of relay l9, upper right hand winding of repeating coil 1, conductor 20,upper winding of retardation coil I6, contacts 2i of'relay l5, conductor22, left hand winding of ringing relay !2, right hand winding of relay[0, and compensating resistance 23 to battery.

Ringing relay l2 and supervisory relay 9, being polar relays, do notoperate with current flowing through their windings in the directiondescribed above.

Line relay ID in operating, completes an obvious circuit for operatingrelay 24 whose contacts 25, in closing, cause relay 26 to operate.

Relay 24 in operating, completes at its contacts 21, a circuit forconnecting condenser 28 in series with the left hand windings or"repeating coil 29 which circuit can be traced from the upper half ofrepeating coil 29, condenser 28, conductor 30, normal contacts 3! ofrelay 32, contacts 2'1 of relay 24, conductor 33 to the lower left handwinding of repeating coil 29.

' Relay 2 in operating also completes a circuit for connectingresistance 34 and condenser 35 in service between the upper and lowerwinding of retardation coil I6 which circuit can be traced from the uperwinding of retardation coil l6, resistance 35, condenser 35, conductor35, contacts 31 of relay 24, conductor 38 to the lower left hand windingof retardation coil l6.

Relay 26 in operating, closes at its contacts 39 an obvious energizingcircuit for the right hand winding of ringing relay l2. ,Relay l2 doesnot operate at this time however, .as the current flowing in thepreviously traced circuit through the middle and left hand windingscreates a magnetic flux whichneutralizes the flux of the right handwinding due to the fact that this winding is connected differentiallywith respectto the middle and left hand windi gs.

Relay 26 in operating, also closes at its contacts 40, a circuit forlighting guard and disconnect lamp 4| at the B operator position whichcircuit can be traced from battery through contacts '40 of relay 26,contacts 42 of relay 43, normal contacts 44 of relay 45, and lamp 4! toground.

When relay 26 operates, it also actuates the start relay 45 associatedwith this particular trunk, the circuit for which can be traced fromground through contacts 4? of relay 26, contacts 48 of relay 49 and thewinding of relay 45 to battery. Relay 46 in operating, opens at itscontacts 50 the energizing circuit for relays corresponding to and tothe left of relay l5 associated with trunks in the same sequence group'andcompletes at its contacts a circuit for operating relay 45, whichcircuit can be traced from battery, through contacts 52 of relay 2%,

winding of relay 55 contacts of relay 46 and chain of contacts 300 ofrelay 301 and 302 of relay 303 to ground.

-Relay 45 in operating, opens at its contacts 53, the energizing circuitfor relays corresponding to and to the right of relay 46 associated withpreceding trunks in the same sequence group such as relays 3!, 302, etc.for example.

Relay 45 in operating, also completes a circuit through its contacts 54to operate relays 55 and 56 whose functions will appear later in thedescription. The circuit for operating relays 55 and 56 can be tracedfrom ground through guard and disconnect lamp 4!, contacts 54 of relay45, winding of relays 55 and 55 to battery. The resistance of relay 56is sufiicient to dim guard and disconnect lamp 4|.

When relay 45 operated, it also closed its contacts 51 and 58 to connectthe trunk to the position circuit POS where the (B) operators telephoneset 0. T. is ultimately connected to the trunk. Therefore since relays45 and 45 have opened the energizing circuits for corresponding relaysto the right and left in other trunks in the same sequence group, it isapparent that no other trunk can be connected to the position until theoperator is released from the present trunk.

Relay 45 in operating, closes at its contacts 55 an energizing circuitfor relay 49 which can be traced from battery through contacts 53 ofrelay 26, winding of relay 4!), normally closed contacts of relay 49,normally closed contacts 55 of relay 43, and contacts 59 of relay 55 toground through a high resistance relay E2 in the position circuit POS.Relay 48, being marginal, does not operate when this circuit is 7closed, due to the high resistance ground. The

position circuit (not shown in detail) however, functions to send twoshort zips of tone back to the'toll office in a well known manner as along as the trunk is connected to the position.

Relay 55, in conjunction with relay 55 provides a circuit for flashingthe guard and dis- "connect' lamp M to indicate to the (B) operator thetrunk to which her telephone set is connected. Relay 55 in operating,completes an obvious circuit for operating relay 54. When relay 54operates and closes its contacts, direct battery is applied to guard anddisconnect lamp 4! over the circuit previously described, lighting 5 thelamp, and at the same time this battery short-circuits the winding ofrelay 56 causing it to release and open its contacts to release relay65. When relay 64 releases, the short circuit is removed from relay 55,guard and dis- 10 connect lamp 4! is dimmed, and the circuit is ready torepeat the cycle of operation just described as long as the trunk isconnected to the position.

When the trunk operator (B) has received the 5 number and partydesignation, if any, of the called subscriber (X), she depresses theproper ringing key, key M for party M etc. and makes the usual busy testby touching the tip of plug 65 to the sleeve of jack E8 and, if shehears no I click, inserts the plug into the jack, whereupon an obviouscircuit is completed through low resistance 61 to operate polar relay 69and marginal relay 68 in parallel.

Relay 68 in operating, opens its normally 5 closed contacts hi, todisconnect the busy test conductor H from the tip, and completes, at itscontacts 72 and I3, circuits for connecting the tip and ring of plug 55to the right hand windings of repeating coil 29. 3[)

Relay 59 in operating, completes an obvious circuit for operating relayl4.

Relay 14 in operating, closes at its contact 15 a circuit for applyingdirect ground to the winding of relay 49 to cause its operation,provided 5 it has not been operated by means of the release key 15 inthe position circuit, which operation will be covered later in thedescription. Relay 49, being marginal, was not operated when connectedto high resistance ground in the po- 40 sition circuit as previouslydescribed. However, when direct ground is applied to the Winding ofrelay 49 by the operation of relay l4, relay 49 operates in a circuitthat can be traced from battery through contacts 52 of relay 2B, winding45 of relay 49, contacts 60 of relay 49, contacts 1'! of relay 43,conductor '18 and contacts 15 of relay 14 to' ground.

Relay 49 in operating, completes, through its continuity contacts 19, aconnection through 50 contact 41 of relay 26 to ground to lock itselfunder control of relay 26.

Relay 49 in operating, also closes a circuit through its contacts 86 foroperating relay 43, which circuit can be traced from ground through 55contacts l 33 of relay T4, conductor 8|, contacts 80 of relay 49 and theleft hand winding of relay 43 to battery.

The operation of relay 49 further performs the function of operatingrelay I5 over a circuit that 60 can be traced from ground throughcontacts 41 of relay 25, contacts E9 of relay 49, conductor 82, windingof relay l5, contacts 83 of relay 84, conductor 35, contacts 85 of relay32, conductor 81,

.contacts 88, of relay 63, contacts 89 of relay l4, c5

conductor 98, and contacts I02 of relay 26 to battery.

Relay 49 in operating also dismisses the position circuit by opening atits contacts 4-3 the energizing circuit for relay 46 causing it torelease.

When relay 46 releases, it closes the chain contacts to ground throughthe relays corresponding to and to the left of relay 45 in thesucceeding associated trunks in the same sequence group to mosaicsprovide energizing paths: for: other calls, and

opens theenergizing circuit for relay 45: causing it to release.

Relay 45 in releasing, opens at itscontacts: 44 the circuit for flashingguard; and disconnect lamp' M, thereby releasing: relay 55 to extinguishthe pilot lamp 63'.

When relay 45 releases, it alsoclosesthe chain contacts to batterythrough the relayscorresponcling to and to the right ofrelay 461 in theprecedingassociated trunk-s in the samesequenoe group, to provideenergizing paths for connecting these relays to the position circuit.

When relay 43 operated. it completed a holding circuit for itself, undercontrol of relay 26 that can be traced from ground; through the righthand Winding of relay 43; make contacts 9! of relay 49, make contacts 92of relay 43 and make contact to of relay 26 to battery. Atthe same timerelay d3 closes at its make contact 93 a holding circuit for itselfunder control of relay T4 that can be traced from batterythrough theleft hand winding of relay 43, make contact 93 of relay 43, conductor.18 and make contact 55. of relay M to ground. This joint. control overrelay 43 is provided to permit the sending of a reorder signal to thetoll operator on a premature disconnect by the trunk operator, to lockin the disconnect signal until the trunk is disconnected from thesubscribers line, and to permit a second call on the same trunk to beconnected to the position circuit before the trunk is disconnected fromthe previous call as will be later described.

When relay I5 operates, it reverses, at its contacts' l4 and 2|, thecurrent flow, over the circuit previously traced, through polarizedrelay 9 at the toll office, causing it to operate and complete, anobvious circuit for applying battery to the ring of jack 3' to give thetoll operator (A) a signal that the trunk has been extended tosubscriber (X) and that the trunk circuit is ready to ring on thesubscribers line;

Relay l5 in operating, also completes, at its; contacts 94', a lockingcircuit. for itself under control of supervisory relay 84; This lockingcircuit: is the same as the". energizing circuit except that contacts 94short circuit contacts 88 of relay 32, thereby removing control of relay455 from relay 32.

When toll operator (A) depresses her ringing key 95ringing current isappliedover the tip and ring of plug 2 to operate relay 96 over acircuit that can be traced from the tip of jack 3, upper left handwinding of repeating coil E, resistance 91, winding of relay 96,condenser 98: andthe lower left hand winding of repeating coil 1 to thering of jack 3.

When relay fifioperates, it completes an obvious circuit for operatingrelay H which in op erating, opens at its contacts 18 the short circuitaround resistance 99 thereby inserting re sistance 99 in the circuit'previously traced through relay i2.

Resistance 99 is of such a value that it reduces the current flowthrough the left hand and middle windings of polar relay i2 so that theflux produced in these windings no longer neutralizes the flux in theright hand winding whereupon and closes itscontacts'to shortcircuitresistance The. energizing circuit for relay Hi6] can be tracedfrom ground through contacts llll of relay 26, contacts of: relay i2,right 'hand winding ofrelay we: and contacts I02 of relay 26 to battery.

When relay 1E9 operates, it closes at its contacts Hi3, an obviouslocking path for itself through resistance EM and at contacts I05, itcompletes a circuit for operating relay 32 that can be traced fromground, through contacts I95of relay I'Oil, right: hand winding of relay32, contacts 9| of'relay 49, contacts 92 of relay 43 and contacts 40 ofrelay 26 to battery.

Ringing filter As reierred to in the statement of features, the presentarrangement utilizes, during the ringing interval, a portion ofrepeating coil 29 as one element of a filter circuit through which" theringing current is passed to the called subscribers line 16'.

Specifically, certain changes are effected, as will later be described,whereby the left hand windings of repeating coil 29 are opened, toimprovetheimpedance characteristics of the coil, 3

condenser ill! and resistance I08 normally connectedbetween the righthand windings of repeating coil 29. are short circuited, condenser 09is. connected across the ringing leads and the source ofringing supplyis connected in series with the right hand windings of repeating, coil29, the tip. and ring of the trunk, and the subscribers line, therebyintroducing a filter in the ringing circuit which offers very littleimpedance to the low frequency ringing current, but presents a highimpedance to. the higher harmonics of, the ringing supply since thesehigher frequencies are objectionable when heard in the subscriber'sreceiver.

Relay 32 in operating, opens at its. contacts 3 F the circuit that .wasclosed by contact 21 of relay 24 to connect condenser 28 between theleft hand windings. of repeating coil 29, and at its contact IIIbridgesresistance HI across condenser 28 to discharge it. This dischargepath, which is provided to leave the condenser in a discharged conditionso that it will not introduce a click when it is reconnected. to thetrunk, can be traced from condenser 28, conductor 39, contact H0 ofrelay 32, conductor H2 and resistance HI back to condenser 28.Resistance HI provides protection for contacts H 0 of relay 32 againstexcessive currents caused by the discharge of condenser 28.

As mentioned ,in the preceding reference to the ringing circuit filter,one reason for opening the connection between the left hand windings ofrepeating coil 29 is to improve the impedance characteristics of thecoil when used asf a filter. This also prevents clicks from beingtransmitted back over the trunk to the toll operator during theestablishment and breakdown of the filter circuit.

Re1ayt2 in operating, also opens at its contacts l l3, an obvious shortcircuit, over conductors IM and H5, around resistance I08 so that theresistance may serve later as a protection for contacts I I6 and II? ofrelay II8.

When relay 32 operated, it opened at its contacts 86 the energizingcircuit for relay I5 but that relay remains operated through its lockingcontacts 94. 7

Relay 32 in operating further performs the function of completing theenergizing circuit for relay H0 which circuit can be traced from ground,through contacts I05 of relay I00, contacts II9 of relay 32, conductorI20, left hand winding of relay II8, contacts 88 of relay 60, contacts89 of relay I4, conductor 90 and contacts I02 of relay 26 to battery.

Relay III! in operating, closes at its contacts I 2I, an obvious circuitover conductor I22 for holding relay 32 operated until relay I I8releases. This is necessary to maintain a predetermined sequence ofoperation in the restoration of the repeating coil to its normalfunction as part of the speech transmission circuit after it has servedits purpose as a filter.

Relay H8 in operating also disconnects at its contacts I23 and I24, thecircuit for connecting supervisory relay 94 across the tip and ring ofplug 65 and at contacts II6 and I I1, short circuits condenser I07 inseries with resistance I08. This short circuit connects the two righthand windings of repeating coil 29 together to for the inductive portionof the ringing filter. The short circuit also discharges condenser I01through resistance I03 sothat it will be in a discharged condition whenit is reinserted between the right hand windings of repeating coil 29 atthe conclusion of ringing thereby eliminating the possibility ofproducing a click in the subscriber set at the moment it is inserted inthe circuit.

Relay II8 further performs the function of completing the energizingcircuit for relay 7 I25 which circuit can be traced from ground throughcontacts I05 of relay I00, contacts I I9 of relay 32, conductor I20,contacts I26 of relay II8, upper winding of relay I25, contacts 88 ofrelay 68, contacts 89 of relay I4, conductor 90 and contacts I02 ofrelay 26 to battery.

completes the energizing circuit for relay I28 which circuit can betraced from ground through contacts I21 of relay I 25, contacts I29 ofkey (M) or I30 of key (J) and the winding of relay I20 to battery.

Relay I25 in operating also completes at its contacts I3I, alocking'circuit for itself under control of tripping relay I32 that canbe traced from ground through contacts. I33 of relay I4, contacts I3I ofrelay I25, lower winding of relay I25, contacts of relay I32, contactsI34 of relay I5, contacts 88 of relay 68, contacts 89 of relay 7G,conductor 90 and contacts I02 of relay 26 to battery.

Contacts -I3I and I35 of relay I25 also complete a circuit for shortcircuiting the winding of relay I00 to cause its release since theringing relay train has been setup at this time. The short circuitingpath can be traced from ground through contacts I33 of relay 79,contacts I3I and I35 of relay I 25, conductor I35, resistance I04,contacts I 03 of relay I00 and contacts I02 of relay 25 to battery.Resistance I00 is provided to permit the application of ground to bothends of the left hand winding of relay I00 to cause its release withoutblowing the fuse in the battery lead.

Relay I25 in operating, further performs at its contacts I91 thefunction of opening the short circuit around condenser I09 andresistance I38 while at its contacts I59 it applies ringing ground toconductor I39. Condenser I09 is the second element of the ringing filterpreviously referred to, the other element being the right hand windingsof repeating coil 29. Resistance I39 is provided to protect contacts I31of relay I25 from excessive current fiow that occurs when relay I25releases and completes the discharge circuit for condenser I09 throughresistance I38, since the condenser will be in a charged condition atthe time the relay releases.

Relay I25 in operating also connects, at its contacts I 4|, the selectedringing potential to the upper right hand terminal of repeating coil 29.

When relay I25 operates, its contacts I42 complete a circuit fromcondenser I49 to conductor 20 to pass a tone to the toll operator toindicate that ringing current is being applied to the subscribers line.

When relay I00 releases, it opens, at its contacts I05, the energizingcircuit for relays 32, H8, and I25. These relays remain operated howeveron their holding windings.

Let us assume that the called subscriber is party M on line I96, andthat the trunk operator (B) has depressed the M ringing key therebyoperating relay I20 over the circuit previously traced which reverses atits contacts I44, I05, I46 and I0? the connections between conductorsI39 and IE8 and the tip and ring of plug 65.

Relay I28 in operating also. completes a circuit for applying positivepulsating current to the tip of plug 65 and ringing ground to the ringof plug 55. This circuit can be traced from positive pulsating currenton conductor 149, contacts I50 of ringing key M, contacts I5I of relayI28, winding of tripping relay I32, contacts I52 of relay I5, contactsI4I of relay I25, upper right hand winding of repeating coil 29,contacts H9 and II! of relay II8, lower right hand winding of repeatingcoil 29, contacts I3 of relay 68, contacts I45 of relay I28, contactsI53 of relay I4, tip of plug 55, over line I06 through the subscribersset and back to the ring of plug 55, contacts M? of relay I28, andcontacts I 40 of relay I 25 to ringing ground. The windings of repeatingcoil 29 and condenser I09 form the ringing filter previously described.

If the called subscriber is party J instead of M, the trunk operatoroperates key J and its contacts I30 and I54 function to complete thecircuits previously described as being completed by contacts I29 and I50of key M with the exception that negative pulsating current is suppliedfrom conductor I55 rather than the positive pulsating current fromconductor I 39.

If the W party is desired, the ,W key is operated. this condition andnegative pulsating current is applied directly to the tripping relay I32from conductor I55 through contact I56 of key (W), whence it follows thecircuit previously traced to contacts I45 and I44 of relay I29 where thenegative pulsating current is connected to the ring of plug 65 andringing around to the tip of the plug.

When the called subscriber is party R the 65 Relay I29 does not operateunder trunk operator presses key R and positive pulsating current isconnected from conductor I49 through contact I51 to the tripping relaywhence it follows the circuit described for party W.

If the called subscriber has a direct line, the trunk .operatorpresseseither the R :or W key, dependent upon the operating practice decidedupon.

When the called subscribers receiver is removed from the switchhook,tripping relay I32 operates in the usual manner due to the increasedcurrent flowing through its winding.

Relay I32 in operating, opens at its contacts the circuit for holdingrelay I25 operated, causing it to release.

:Relay I25 in releasing, opens its contacts .IM to disconnect theringing supply fromthe called line, thereby releasing relay I32, and atcontacts I40 it disconnects ringing ground from conductor I39 and at itscontacts I31 short circuits condenser I99 and resistance I38. Condenser=I09 discharges through resistance I38.

Relay I25 in releasing, also opens, at its contacts I42, the circuit fortransmitting tone to the toll operator as an indication that ringingcurrent has been removed from the subscriber's line.

Relay I25 in releasing, furtherperiorms the function of opening at itscontacts I2I the energizing circuit of relay I28 (if key J or M isoperated), and the holding circuit for relay I I8.

When relay 28 releases, it restores at its contacts I44 and I45, thenormal connections between repeating coil 29 and plug 69.

Relay II8 in releasing, opens at its contacts IIB and Ill, the shortcircuit on resistance I08 and condenser I91 and at its contacts I23 andI24 completes a circuit for operating relay 9 4, which circuit can betraced from battery through resistance I59, left hand winding of relay84, contact I24 of relay IIB, lower right hand winding of repeating coil29, make contact I3 of relay 58, contact I45 of relay I28, ring of plug80, ring of jack GI, over line I06 through the subscribers set and backto the tip of jack GI, tip of plug 69, contact I53 of relay I4, contactI44 oi relay I28, break contact I3l of relay I25, upper right handwinding of repeating coil 29, contact I23 of relay II8, right handwinding of relay 84 and resistance I59 to ground.

Relay H8 in releasing also opens at its contacts I2I the circuit, overconductor I22, for holding relay 32 operated, thus causing its release.

Relay 32 in releasing, applies, at its contacts II3, a short circuitaround protective resistance I93 thereby leaving condenser IO'I bridged,be-

' tween the upper and lower right hand windings of repeating coil 29 toestablisha normal talking condition.

Relay 32 in releasing, at its contacts IIII, disconnects resistance IIIfrom condenser 23 and bridges the condenser between the left handwindings of repeating coil 29 thereby completing the talking connectionbetween the toll operator (A) and subscriber (X).

Relay 32 in releasing, also restores the path for energizing relay I5.

Relay 84 in operating, opens at its contacts the energizing circuit forrelay I5 causing it to release. I

Relay I5 in releasing, restores at its contacts I4 and 2|, the normaldirection of current flow through relay '9 causing it to release anddisconnect battery from the toll cord supervisory 49 until relay43rreleases. =.49 from operating ii a second call is originated relay toextinguish thetoll .operators supervisorylamp .as anindication. thatzthe.called subscriber had :answered.

' Relay I5 in releasing, also .connectsa source ofpulsatingcurrent tocontacts I69 .of relay-I25 5. to. provide a .continuoussringingsupply'for recall purposes to .be later described.

When the receiver at the called stationdscreplaced on the ,switchhook,supervisory relay :84 releases, reestablishing the energizing circuitfor 10 relay I5 over the path previously traced.

.Relay I15 incoperatingreverses atits contacts I4 .and 2.I, .theicircuit through relay 9 causing it to reoperate and. apply .battery tothe toll cord supervisory relay, causing that relay to operate 15 andlight the toll supervisory lamp as an. indication that the calledsubscriber has hungnp.

Relay 13 in .operating, .also completesat "its contacts94 its originalholdingcircuit previously described. 20

When the toll :operator (A) disconnects the toll cord .2 from jack .3,relay 4 releases sand disconnects the battery asupplyfrom the ring .orjack 3.

Relay 4 in releasing, opens at its contacts 5 25 Relay 24 in releasing,also opens, at itscontacts 21, .the circuit over conductors 39 -and 33for bridging condenser .28 between the left hand windings of repeatingcoil.29.

Relay 24 in releasing, further performs the function of opening, at itscontacts 25, theener- 40 gizing circuit for relay ZBcausing its release.

.Relay 26 in releasing, opens at its contacts 40, the holding circuitthrough the right handwinding of -relay'43 and completes a circuit forlighting guard and disconnect lamp :4I, as a:disconnect signal for the(B) operator. The circuit for lighting lamp M, can be traced frombattery through contacts -I6I of relay 26, conductor I62, contacts I63of relay l4,conductor I64, contacts 44 of relay 55 andlamp'4l to:ground.

Relay 26 in releasing, also opens at'its contacts 39, the energizingcircuit for the right hand winding of relay I2 causing that relay torelease. Relay I2 operated when the circuit through the left hand andmiddle windings was opened at the toll board by therelease of relay 4,but, due to the slow acting characteristics of relays .32 and II 8, did*not have time to set up the ringing condition before-its release.

Relay 26 in releasing further performs the function of opening at itscontacts 52 and 41 the energizing circuit for relay 49 thereby causingthat relay to release.

Relay 43 does not release when 'the circuit through its right handwinding is opened, provided the trunk is connected to the .subscribersline, since it is held operated .byits left .hand winding .under controlof sleeve relay 14. This sleeve controlof the release of .relay 43-is;provided to lock in the disconnect signal untilthe trunkisdisconnected from ,the subscribers .line by holding opentheenergizingcircuitz-for relay This -prevents relay by the toll operatorbefore the trunk is 'disconnected from the first called subscribersline.

When the trunk operator disconnects plug 65 from jack 66, relays 68 and69 release.

When relay 68 releases, it prepares, in part, a circuit for sending arecall signal to the toll operator which function will be laterdescribed in'detail.

. Relay 69 in releasing, opens at its contacts the energizing circuitfor relay 14 causing its release.

. When relay 14 releases, it opens, at its contacts I63, the circuitpreviously traced for lighting guard and disconnect lamp 4|, and, at itscontacts I65, connects the tip of plug 65 to the busytest lead 1Ithrough contacts 16 of relay 68.

'Relay 14 in releasing also opens at its contacts.15 the holdingcircuit, over conductor 18, for relay 43 causing it to release,restoring the circuit to normal.

. If the (B) operator disconnects the trunk from the subscriber's linebefore the disconnect signal is set at her position, the circuit isarranged to give the toll operator a reorder signal as follows:

Removal of plug 65 from jack 66, While the toll cord is connected to thetrunk, causes the release of relays 84 (if operated) 68 and 69 and relay69 in releasing, releases relay 14.

When relay 14 releases it opens at its contacts 15 the circuit throughthe left. hand winding of relay 43 over conductor 18, but relay 43 doesnot release as it is held operated by its right hand Winding undercontrol of relay 26.

Relay 14 in releasing, completes a circuit for flashing relay I5 thatcan be traced from battery, interrupter I66, contacts I61 of relay 14,conductor I68, contacts I69 of relay 43, conductor 85, contacts 83 ofrelay 84, winding of relay I5,Vconductor 82, contacts 19 of relay 49 andcontacts 41 of relay 26 to ground.

Relay I5 follows the interrupter and each time it operates, reverses,through its contacts I4 and 2|, the current flow through polarized relay9 at the toll office causing that relay to intermittently light thesupervisory lamp in the toll cord as previously described.

Relay I5 in operating cannot look at this time as the circuit forsupplying locking battery is opened at contacts 88 of relay 68 andcontacts .89of relay 14.

The toll operator answers the recorder signal by withdrawing plug 2 fromjack 3 and then originates a new call. Withdrawal of plug 2 causesrelays I6, 24 and 26 to release as previously described for thedisconnection of a regular call. Relay 26in releasing opens at itscontacts 46 the holding circuit for, relay 43 causing it to release andreturn to normal with the other relays. The new call proceeds aspreviously described.

Should the toll operator originate a second call before the (B) operatordisconnects the trunk from the subscriber's line of the previous call,the circuit functions in the following manner.

Upon the insertion of plug 2 in jack 3, the

call progresses as on a regular call until start relay 46 operates. Whenrelay 46 operates it performs the functions previously described exceptthat the connection to ground through the high resistance relay in theposition circuit is now traced from battery through contacts 52 of relay26, resistance I10, contacts I1I of relay 43, which relayis operatedsincethe trunk has not been disconnected from line I66, contacts 59 ofrelay 45 and winding of relay 62 to'ground. Pilct lamp 63 lights andguard and disconnect lamp 4i flashes to indicate the trunk that isconnected to the position. Under this condition, 1. e., the trunkconnected to line I66, the (B) operator cannot disconnect the trunk fromthe position circuit by the operation of the release key 16 until thetrunk is disconnected from the line, thereby releasing relays 68, 69,14, and 43. Relay 43 in releasing substitutes, at its continuitycontacts 6| and HI, relay 49 for resistance I10 and. the circuit thenassumes the condition of a regular call.

The trunk operator may disconnect the position circuit from the trunkcircuit by operation of the position release key 16 unless the callbeing answered is a second call", that is, one that has been connectedto the position circuit before the trunk operator has disconnected fromthe previous call. In that case the position cannot be disconnecteduntil the connection to the subscribers line has been taken down aspreviously described.

Operation of release key I6 causes the position circuit to function, inthe usual manner, and lower the resistance in series with relay 49 overthe circuit previously traced, thereby causing relay 49, which ismarginal, to operate and disconnect the trunk from the position circuit.

Emergency listening If the position circuit is not functioning, thetrunk operator completes the connection to her telephone set byinserting plug 65 in jack I12 of line I13 to the emergency listeningcircuit EL. The insertion of plug 65 in jack I12 completes an obviousenergizing circuit for relays 68 and 69. Relay 69 being polarized, doesnot operate since the current flow is in the nonoperate direction, butmarginal relay 68 does operate and completes at its contacts 12 and 13,a circuit for connecting repeating coil 29 through to the emergencylistening circuit. Battery is then connected through resistance I58, theleft hand winding of relay 84, contacts I24 of relay II8 lower righthand winding of repeating coil 29, contacts 13 of relay 68, contacts I46of relay I28, the ring of plug 65 and the ring of jack I12 to the ringof line I13 where it causes the emergency listening circuit to functionand connect the trunk to the (B) operator's telephone circuit. Since theoperation of the emergency listening circuit is not a part of thisdisclosure its operation will not be described in detail. The emergencylistening circuit is held over the circuit just traced to the ring ofline I13 where it passes through the Winding of a relay in the emergencylistening circuit to the tip of line I13, tip of jack I12, tip of plug65, contacts I65 of relay 14, contacts 12 of relay 68, contacts I44 ofrelay I28, contacts I 31 of relay I25, upper right hand winding ofrepeating coil 29, contacts I23 of relay II8, right hand winding ofrelay 84 and resistance I59 to ground. The toll operator is circuitfunctions.

the toll operator as previously described.

When the trunk operator removes trunk plug 65 from jack I12, relay 68releases and the trunk is ready for connection to thesiibscribers line.

Toll operator recall If the (B) operator desires to recall the tolloperator on a standard basis, the trunk cord plugis inserted in jack I14to connect the trunk to the reorder circuit R0 over line I15. When theplug 65 is inserted in jack I14, ground through high resistance I16 isconnected through the sleeve of jack I14 and plug 65 to relays 68 and 69causing polarized relay 69 to operate and cause the operation of relay14. Marginal relay 58 does notoperate when,connected to a highresistance ground, so that a circuit is completed through its contactsfor flashing the toll operator as soon as the reorder The insertion ofplug 65 in jack I14 causes the reorder circuit to function, in a mannernot shown, to connect interrupted battery to the ring of jack I14 toflash relay I5 thereby causing the toll operator to receive a flashingsupervisory lamp in the manner previously described. The circuit forflashing relay I5 can be traced from interrupted battery on the ring ofjack 11% through the ring of plug 65, contacts I46 of relay I23,contacts I11 of relay 88, contact 83 of relay 84, winding of relay I5,conductor 82, contacts 19 of relay 49 and contacts 41 of relay 25 toground. The toll operator answers such a recall by withdrawing plug 2from jack 3, thereby causing the trunk circuit to function as previouslyde scribed and set up a disconnect signal for the (B) operator. The tolloperator then reinserts plug 2 in jack 3 to reestablish a talkingconnection to the trunk.

Busy back When the (B) operator tests the called subscribers line andfinds it busy, plug 65 is inserted in jack I18, of line I19 to the wellknown busy back circuit BB. This circuit presents the same operatingcharacteristics as the reorder trunk R0, except for the rate offlashing, and the trunk functions as previously described for.reordersignals. The flashing rate of the two circuits is made different toenable the toll operator to distinguish between the signals.

If the called subscriber is located at a. PBX it maybe necessary for thetoll operator to recall the PBX operator to pass additional information,in which case she rings onthe trunk in the usual manner. The trunkfunctions as on a regular call, and relays 96, I9, I2, I00, 32, H8, andI25 operate to set up the ringing conditions including the ringingfilter. At this time, however, relay I5 is not operatedand consequentlypulsating current is applied to the line at its contacts I69 over thecircuit previously traced for ringing from these contacts. Relay I32 isnot connected inthe circuit and the holding circuit throughthelowerwinding of relay I25 is held open at contacts I34, of relay I5 to givethe toll operator direct control of the continuous ringing supply to thecalled line. The various relays in the ringing chain therefore followthe operation of the ringing key at the toll oflice.

What is claimed is:

1. In a telephone trunking system a first switchboard, a secondswitchboard, a trunk line therebetween, a called line accessible to saidtrunk at the second switchboard, means to connect said trunk line to thecalled line a source of ringing current and means at the secondswitchboard for controlling the connection of said ringing source to thecalled line, said means comprising a differential relay controlled overthe trunk conductors from the first switchboard.

2. In a telephone system, a first operators position, a second operatorsposition, a trunk line therebetween, a link circuit at the firstposition adapted tobe connected to said trunk, a first circuit includinga relay and a source of current bridged across the trunk at the secondposition, means at the first position responsive to connectiondf saidlink with said trunk to complete said first circuit over said trunk, anauxiliary circuit also including said current source, an auxiliarywinding on said relay included in said auxiliary circuit, said auxiliarying signal transmitted to said trunk from said link circuit to reducethe current in said first circuit sufficiently to permit said relay tooperate on its auxiliary circuit winding, a source of ringing current,and means responsive to operation of said relay to connect said ringingsource to said trunk circuit.

3. In a I telephone system, a first operator's position, a secondoperators position, a trunk line therebteween, a link circuit at thefirst position adapted to be connected to said trunk line, subscriberslines at the second position adapted to be connected to by said trunkline, a circuit including a source of direct current connected acrosssaid trunk line at the second position, a low resistance direct currentpath adapted to be completed across said trunk line at the firstposition responsive to connection of said link circuit with said trunk,a source of ringing current at the second position, means for applyingsaid ringing current to said trunk line to ring on .a connected one ofsaid subscribers lines, a multiple wound relay at the second positionadapted to control said ringing means, said relay having two windings ina series aiding relation, included in said direct current circuit andhaving a third winding included in a second circuit which also includessaid direct current source, said third winding being so arranged as toneutralize the effect of said first two windings when said lowresistance path is connected across the trunk line at the firstposition, and means at the first position responsive to application of.a ringing signal to the trunk thereat to increase the resistance ofsaid direct current path sufficiently to reduce' scribers lines at thesecond position adapted to be connected to by said trunk line, a firstcircuit including a relay and a. source of current bridged across thetrunk at the second position, means at the first position responsive toconnection of said link circuit with said trunk to complete said firstcircuit, an auxiliary circuit also including said current source, anauxiliary winding on said relay included in said auxiliary circuit, saidauxiliary winding being so arranged as to neutralize the effect on saidrelay of the current in said first circuit caused by connection of saidlink to said trunk, means responsive to a ringing signal transmitted tosaid trunk from said link circuit to reduce the current in said relaysufiiciently to permit it to operate on its auxiliary winding, 2. sourceof ringing current, a repeating coil interposed in said trunk line andmeans jointly responsive to operation of said relay and connection ofsaid trunk line to one of said subscribers line to connect said ringingsource through the repeating coil winding, nearest the subscribers line,to said line.

'5. In a telephone trunking system, first and second operatorspositions, a trunk line therebetween, a repeating coil interposed insaid trunk line at the second position, a called line connected to saidtrunk line at the second position, a source of ringing current and meansat the second position for connecting said ringing source and saidcalled line through the called line side of the repeating coil, saidmeans comprising a differential relay controlled over the trunk linefrom the first position.

6. In a telephone system, a switchboard, a called line appearingtherein, a link circuit terminating thereat and connected to said line,a repeating coil interposed in said link circuit, a source of ringingcurrent and means controlled over said link circuit for connecting saidsource to the called line through the called line side of the repeatingcoil.

7. In a. telephone system, a switchboard, a called line appearingtherein, a link circuit terminating in a plug thereat, said plug beingconnected to said line, a repeating coil interposed in said linkcircuit, a source of ringing current, and means for connecting saidsource to the called line through the repeating coil winding nearestsaid line.

8. In a telephone system, a switchboard, a called line appearingtherein, a link circuit terminating in a plug thereat, said plug beingconnected to said line, a repeating coil interposed in said linkcircuit, a source of ringing current, a circuit therefor including afilter, and means for connecting said circuit to the called line throughthe called line side of said repeating coil.

9. In a telephone system, a switchboard, a called line appearingtherein, a link circuit terminating in a plug thereat, said plug beingconnected to said line, a repeating coil interposed in said linkcircuit, a source of ringing current, a circuit therefor including afilter, and means for connecting said circuit to the called line throughthe called line side of the said repeating coil and for opening theother half of said coil.

10. In a telephone system, a switchboard, a called line appearingtherein, a link circuit terminating in a plug thereat, said plug beingcona differential relay having a first winding connected to saidtrunk'line and having a second Winding in a local energizing circuit,arranged to balance said first winding for a certain line current, meansfor charging the line current, and unbalance the relay and permit it tooperate and signal means controlled by operation of said relay.

12. In a signaling system, a relay winding in a closed circuit includinga line and a source of current, a difierential winding on said relayenergized in a local circuit also including said source of current toneutralize the flux due to the line winding and means to change theresistance of said closed circuit to unbalance said relay therebypermitting its operation.

13. In a signaling system, a line, a multiple wound relay having onewinding connected on one end of said line, a source of current in serieswith said winding, means for closing the other end of the line through apretermined resistance, a differential winding on said relay in a localcircuit including said source of current, said diiferential windingbeing so arranged as to neutralize the fiux in said relay due to theline circuit through said resistance, and means for changing the valueof said resistance to unbalance said relay and permit it to operate.

14. In a selective signaling system a transmitting station, a receivingstation, a line therebetween, a source of current, a circuit includingsaid source connected across the line at the receiving station, firstand second relays at the receiving station, each relay having a windingconnected in series in said circuit, means for closing the line at thetransmitting station through a resistance to operate said first relay, adifferential winding on said second relay, an auxiliary circuitincluding said differential winding and said source, means for closingsaid auxiliary circuit upon the operation of said relay, saiddifferential winding being so arranged that, when energized in saidauxiliary circuit, it will neutralize the fiux in the second relay dueto the line current through its other winding in series with saidresistance, and means for increasing the value of said resistance at thetransmitting station thereby causing a flux unbalance in said secondrelay and permitting it to operate.

15. In a telephone exchange system an operators position, a trunk lineterminating thereat, signal control means at said position including adifierential relay having one winding unvariably energized in a localcircuit and a second winding connected to said line, and means forvariably energizing said second Winding over said line to efiectoperation of said relay.

SEWELL W. ALLISON. JAMES B. MCICIM.

